The investigators have design a randomized double blind clinical trial to know the incidence of breaktrough pain (BP) in high-risk of BP parturients (nulliparous with early cervical dilation) comparing two epidural analgesic regimes: programed intermittent epidural boluses versus patient controlled epidural analgesia. The role of the epidural technique (epidural versus combined spinal-epidural) in the incidence of BP will be also evaluated.